implement systemd socket activation

default.nix

@@ -1,10 +1,25 @@
 with import <nixpkgs> {};
-mkShell {
-  nativeBuildInputs = [
-    go
-    vault
+
+buildGoModule {
+  name = "systemd-vault";
+  src = ./.;
+  vendorSha256 = null;
+  checkInputs = [
     python3.pkgs.pytest
     golangci-lint
-    hivemind
+    vault
   ];
+  meta = with lib; {
+    description = "A proxy for secrets between systemd services and vault";
+    homepage = "https://github.com/numtide/systemd-vault";
+    license = licenses.mit;
+    maintainers = with maintainers; [ mic92 ];
+    platforms = platforms.unix;
+  };
 }
+#mkShell {
+#  nativeBuildInputs = [
+#    go
+#    hivemind
+#  ];
+#}

main.go

@@ -20,7 +20,37 @@ type server struct {
 	connectionClosed chan int
 }
 
+func inheritSocket() *net.UnixListener {
+	socks := systemdSockets(true)
+	stat := &syscall.Stat_t {}
+	for _, s := range socks {
+		fd := s.Fd()
+		err := syscall.Fstat(int(fd), stat);
+		if err != nil {
+			log.Printf("Received invalid file descriptor from systemd for fd%d: %v", fd, err)
+			continue
+		}
+		listener, err := net.FileListener(s)
+		if err != nil {
+			log.Printf("Received file descriptor %d from systemd that is not a valid socket: %v", fd, err)
+			continue
+		}
+		unixListener, ok := listener.(*net.UnixListener);
+		if !ok {
+			log.Printf("Ignore file descriptor %d from systemd, which is not a unix socket", fd)
+			continue
+		}
+		log.Printf("Use unix socket received from systemd")
+		return unixListener
+	}
+	return nil
+}
+
 func listenSocket(path string) (*net.UnixListener, error) {
+	s := inheritSocket()
+	if s != nil {
+		return s, nil
+	}
 	if err := syscall.Unlink(path); err != nil && !os.IsNotExist(err) {
 		return nil, fmt.Errorf("Cannot remove old socket: %v", err)
 	}

systemd_sockets.go

@@ -0,0 +1,52 @@
+package main
+
+import (
+	"os"
+	"strconv"
+	"strings"
+	"syscall"
+)
+
+const (
+	// listenFdsStart corresponds to `SD_LISTEN_FDS_START`.
+	listenFdsStart = 3
+)
+
+// Files returns a slice containing a `os.File` object for each
+// file descriptor passed to this process via systemd fd-passing protocol.
+//
+// The order of the file descriptors is preserved in the returned slice.
+// `unsetEnv` is typically set to `true` in order to avoid clashes in
+// fd usage and to avoid leaking environment flags to child processes.
+func systemdSockets(unsetEnv bool) []*os.File {
+	if unsetEnv {
+		defer os.Unsetenv("LISTEN_PID")
+		defer os.Unsetenv("LISTEN_FDS")
+		defer os.Unsetenv("LISTEN_FDNAMES")
+	}
+
+	pid, err := strconv.Atoi(os.Getenv("LISTEN_PID"))
+	if err != nil || pid != os.Getpid() {
+		return nil
+	}
+
+	nfds, err := strconv.Atoi(os.Getenv("LISTEN_FDS"))
+	if err != nil || nfds == 0 {
+		return nil
+	}
+
+	names := strings.Split(os.Getenv("LISTEN_FDNAMES"), ":")
+
+	files := make([]*os.File, 0, nfds)
+	for fd := listenFdsStart; fd < listenFdsStart+nfds; fd++ {
+		syscall.CloseOnExec(fd)
+		name := "LISTEN_FD_" + strconv.Itoa(fd)
+		offset := fd - listenFdsStart
+		if offset < len(names) && len(names[offset]) > 0 {
+			name = names[offset]
+		}
+		files = append(files, os.NewFile(uintptr(fd), name))
+	}
+
+	return files
+}

tests/test_service.py

@@ -3,6 +3,7 @@ import subprocess
 from dataclasses import dataclass
 from command import Command, run
 from pathlib import Path
+import time
 
 import string
 import random
@@ -27,14 +28,17 @@ def random_service(secrets_dir: Path) -> Service:
     return Service(service, secret_name, secret_path)
 
 
-def test_service(systemd_vault: Path, command: Command, tempdir: Path) -> None:
+def test_socket_activation(
+    systemd_vault: Path, command: Command, tempdir: Path
+) -> None:
     secrets_dir = tempdir / "secrets"
+    secrets_dir.mkdir()
     sock = tempdir / "sock"
-    command.run([str(systemd_vault), "-secrets", str(secrets_dir), "-sock", str(sock)])
-    import time
+
+    command.run(["systemd-socket-activate", "--listen", str(sock), str(systemd_vault), "-secrets", str(secrets_dir), "-sock", str(sock)])
 
     while not sock.exists():
-        time.sleep(1)
+        time.sleep(0.1)
 
     service = random_service(secrets_dir)
     service.secret_path.write_text("foo")
@@ -59,6 +63,15 @@ def test_service(systemd_vault: Path, command: Command, tempdir: Path) -> None:
     assert out.stdout == "foo"
     assert out.returncode == 0
 
+
+def test_blocking_secret(systemd_vault: Path, command: Command, tempdir: Path) -> None:
+    secrets_dir = tempdir / "secrets"
+    sock = tempdir / "sock"
+    command.run([str(systemd_vault), "-secrets", str(secrets_dir), "-sock", str(sock)])
+
+    while not sock.exists():
+        time.sleep(0.1)
+
     service = random_service(secrets_dir)
 
     proc = command.run(